Method for generating downlink frame, and method for searching cell

ABSTRACT

The present invention relates to a method of generating a downlink frame. The method of generating the downlink frame includes: generating a first short sequence and a second short sequence indicating cell group information; generating a first scrambling sequence and a second scrambling sequence determined by the primary synchronization signal; generating a third scrambling sequence determined by the first short sequence; scrambling the first short sequence with the first scrambling sequence and scrambling the second short sequence with the second scrambling sequence and the third scrambling sequence; and mapping the secondary synchronization signal that includes the scrambled first short sequence and the scrambled second short sequence to a frequency domain.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of PCT application No.PCT/KR2008/004221, filed on Jul. 18, 2008, which claims priority to andthe benefit of Korean Patent Application No. 10-2007-0072837 filed onJul. 20, 2007, Korean Patent Application No. 10-2007-0083915 filed onAug. 21, 2007, Korean Patent Application No. 10-2008-0042444 filed onMay 7, 2008, Korean Patent Application No. 10-2008-0063387 filed on Jul.1, 2008, the entire contents of which are incorporated herein byreference.

BACKGROUND

(a) Field

The present invention relates to a method of generating a downlink frameand a method of searching cells. More particularly, the presentinvention relates to a method of generating a downlink frame and amethod of searching cells by using the downlink frame in an orthogonalfrequency division multiplexing (OFDM)-based cellular system.

(b) Description of the Related Art

In a direct sequence code division multiple access (DS-CDMA) system, asequence hopping method is applied to a pilot channel so as to acquirecell synchronization and unique cell identification information.According to the sequence hopping method, a mobile station easilyperforms a cell search without a separating synchronization channel byintroducing a sequence hopping technology to the pilot channel. However,in the OFDM system, a number of channels that are capable of beingdistinguished by a frequency domain in a symbol duration of one timedomain is greater than that of those that are capable of beingdistinguished by a spread of CDMA in the symbol duration of one timedomain. Accordingly, when only the time domain is used, resources may bewasted in terms of capacity. For this reason, it is inefficient todirectly apply the sequence hopping method to the time domain of thepilot channel in the OFDM-based system. Therefore, it is preferable tosearch the cell by efficiently using received signals in both timedomain and frequency domain.

An example of an existing technology for searching a cell in the OFDMsystem includes a method that allocates synchronization information andcell information by dividing one frame into four time blocks. For theabove-described method, two frame structures have been proposed. In afirst frame structure, synchronization identification information, cellgroup identification information, and cell unique identificationinformation are allocated to four time blocks, respectively. In a secondframe structure, the synchronization identification information and thecell unique identification information are allocated to a first timeblock and a third time block, and the synchronization identificationinformation and the cell group identification information are allocatedto a second time block and a fourth time block.

According to the first frame structure, since the symbol synchronizationis acquired in only the first time block, it is impossible for themobile station to conduct rapid synchronization acquisition within aprescribed 5 ms during power-on or handover between heterogeneousnetworks. In addition, it is difficult to acquire diversity gain byaccumulating synchronization identification information so as to conductrapid synchronization acquisition.

According to the second frame structure, the unique cell identificationinformation or the cell group identification information is correlatedalong with the synchronization acquisition. Therefore, a cell searchingprocess is complex and a rapid cell search is difficult.

As an example of another technology for searching the cell, a method ofacquiring the synchronization and searching the cell by using a separatepreamble has been proposed. However, this method cannot be applied to asystem in which the preamble does not exist. Moreover, the preamble isdisposed in front of the frame. Accordingly, in a case in which themobile station would like to acquire the synchronization at a timelocation that is not the start of the frame, there is a problem in thatit must wait for the next frame. Particularly, the mobile station shouldacquire initial symbol synchronization within 5 msec during the handoveramong a GSM mode, a WCDMA mode, and a 3GPP LTE mode, but may acquire thesynchronization by a frame unit. For this reason, in some cases, themobile station cannot acquire the initial symbol synchronization within5 msec.

As an example of another technology for searching a cell, there is amethod of searching the cell by allocating two short sequences to asecondary synchronization channel and by mapping cell ID information toa combination of two short sequences. According to this method, sinceinterference occurs between cells when the same short sequence isallocated to sectors adjacent to each other, there is a problem in thatperformance in searching cells is reduced.

SUMMARY

The present invention has been made in an effort to provide a method ofgenerating a downlink frame that is capable of averaging interferencebetween sectors and a method of efficiently searching cells by receivingthe downlink frame.

An exemplary embodiment of the present invention provides a method ofgenerating a downlink frame, including: generating a first shortsequence and a second short sequence indicating cell group information;generating a first scrambling sequence and a second scrambling sequencedetermined by the primary synchronization signal; generating a thirdscrambling sequence determined by the first short sequence; scramblingthe first short sequence with the first scrambling sequence andscrambling the second short sequence with the second scrambling sequenceand the third scrambling sequence; and mapping the secondarysynchronization signal that includes the scrambled first short sequenceand the scrambled second short sequence to a frequency domain.

Another embodiment of the present invention provides an apparatus forgenerating a downlink frame including: a sequence generating unit thatgenerates a first short sequence and a second short sequence indicatingcell group information, a first scrambling sequence and a secondscrambling sequence determined by the primary synchronization signal,and a third scrambling sequence determined by the first short sequence;and a synchronization signal generating unit that scrambles the firstshort sequence with the first scrambling sequence and scrambles thesecond short sequence with the second scrambling sequence and the thirdscrambling sequence, and then generates a secondary synchronizationsignal including the scrambled first short sequence and the scrambledsecond short sequence respectively.

Yet another embodiment of the present invention provides a method ofsearching a cell, including: receiving a downlink frame including aprimary synchronization signal and a secondary synchronization signal;and estimating the information of a cell by using the primarysynchronization signal and the secondary synchronization signal. In thiscase, the downlink frame includes a first short sequence and a secondshort sequence indicating cell group information, a first scramblingsequence and a second scrambling sequence determined by the primarysynchronization signal, and the third scrambling sequence determined bythe first short sequence.

Still another embodiment of the present invention provides an apparatusfor searching a cell, including: a receiving unit that receives adownlink frame including a primary synchronization signal and asecondary synchronization signal; a cell group estimating unit thatidentifies a cell group to by using the secondary synchronizationsignal; and a cell estimating unit that identifies a cell in the cellgroup by using the primary synchronization signal. In this case, thedownlink frame includes a first short sequence and a second shortsequence indicating cell group information, a first scrambling sequenceand a second scrambling sequence determined by the primarysynchronization signal, and the third scrambling sequence determined bythe first short sequence.

Still another embodiment of the present invention provides a recordingmedium that records a program for executing the method of generating thedownlink frame. The recording medium includes: generating a first shortsequence and a second short sequence indicating cell group information;generating a first scrambling sequence and a second scrambling sequencedetermined by the primary synchronization signal; generating a thirdscrambling sequence determined by the first short sequence; scramblingthe first short sequence with the first scrambling sequence andscrambling the second short sequence with the second scrambling sequenceand the third scrambling sequence; and mapping the secondarysynchronization signal that includes the scrambled first short sequenceand the scrambled second short sequence to a frequency domain.

According to the above-mentioned present invention, interference betweensectors can be reduced by scrambling the short sequences due to thescrambling sequences, thereby improving performance for searching cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a downlink frame in an OFDM systemaccording to an exemplary embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of a secondarysynchronization channel when two sequences are mapped to a frequencydomain in a localization form.

FIG. 3 is a diagram illustrating a configuration of a secondarysynchronization channel when two sequences are mapped to a frequencydomain in a distribution form.

FIG. 4 is a block diagram of an apparatus for generating a downlinkframe according to the exemplary embodiment of the present invention.

FIG. 5 is a flowchart illustrating a method of generating a downlinkframe according to the exemplary embodiment of the present invention.

FIG. 6 is a diagram illustrating a first method of generating asecondary synchronization signal according to the exemplary embodimentof the present invention.

FIG. 7 is a diagram illustrating a second method of generating asecondary synchronization signal according to the exemplary embodimentof the present invention.

FIG. 8 is a diagram illustrating a third method of generating asecondary synchronization signal according to the exemplary embodimentof the present invention.

FIG. 9 is a block diagram of an apparatus for searching cells accordingto an exemplary embodiment of the present invention.

FIG. 10 is a flowchart illustrating a method of searching a cellaccording to a first exemplary embodiment of the present invention.

FIG. 11 is a flowchart illustrating a method of searching a cellaccording to a second exemplary embodiment of the present invention.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. In addition, parts that are irrelevant to the description ofthe present invention are omitted in the drawings, to clarify thepresent invention. Like reference numerals designate like elementsthroughout the specification.

Throughout the specification, unless explicitly described to thecontrary, the word “comprise” and variations such as “comprises” or“comprising” will be understood to imply the inclusion of statedelements but not the exclusion of any other elements. In addition, theterm “unit” described in the specification means a unit for processingat least one function and operation, and can be implemented by hardwarecomponents or software components and combinations thereof.

First, referring to FIGS. 1 to 3, a downlink frame of an OFDM system anda configuration of a synchronization channel according to an exemplaryembodiment of the present invention will be described.

FIG. 1 is a diagram illustrating a downlink frame of an OFDM systemaccording to an exemplary embodiment of the present invention. In FIG.1, a horizontal axis represents a time axis and a vertical axisrepresents a frequency axis or sub-carrier axis.

As shown in FIG. 1, a downlink frame 110 according to the exemplaryembodiment of the present invention has a time duration of 10 msec andincludes ten sub-frames 120. Each sub-frame 120 has a time duration of 1msec and includes two slots 130. Each slot 130 includes six or sevenOFDM symbols. The length of a cyclic prefix in a case in which one slotincludes six symbols is greater than that of a cyclic prefix in a casein which one slot includes seven symbols.

As shown in FIG. 1, the downlink frame 110 according to the exemplaryembodiment of the present invention includes two synchronizationdurations 140 in total, including synchronization durations 140 in slotNo. 0 and slot No. 10, respectively. However, it is not necessarilylimited thereto. The downlink frame 110 may include a synchronizationduration in any slot, and may include one synchronization duration orthree or more synchronization durations. Since the length of the cyclicprefix may be different in each slot, it is preferable that thesynchronization duration is located at an end of the slot.

Each slot includes a pilot duration.

The synchronization duration according to the exemplary embodiment ofthe present invention includes a primary synchronization channel and asecondary synchronization channel, and the primary synchronizationchannel and the secondary synchronization channel are disposed so as tobe adjacent to each other in view of time. As shown in FIG. 1, theprimary synchronization channel is located at the end of the slot, andthe secondary synchronization channel is located right ahead of theprimary synchronization channel.

The primary synchronization channel includes a primary synchronizationsignal having information for identifying symbol synchronization andfrequency synchronization, and some information for cellidentification(ID). The secondary synchronization channel includes asecondary synchronization signal having remaining information for thecell ID, and information for identifying frame synchronization. A mobilestation identifies the cell ID of cell by combining the cell IDinformation of the primary synchronization channel and the cell IDinformation of the secondary synchronization channel.

For instance, assuming that the total number of cell IDs is 510, ifthree identification sequences are allocated to the primarysynchronization channel to divide all 510 cell IDs into three groups andif 170 sequences are allocated to the secondary synchronization channel(3×170=510), the information on all of the 510 cell IDs can berepresented.

Another method is that the 510 cell IDs are divided into 170 groups byusing 170 secondary synchronization signals that are allocated to thesecondary synchronization channel, and information on cell IDs in eachcell group can be represented by three primary synchronization signalsthat are allocated to the primary synchronization channel.

Since the secondary synchronization channel includes the information foridentifying the frame synchronization as well as information for thecell ID, two secondary synchronization channels included in one frameare different from each other.

FIG. 2 is a diagram illustrating a configuration of a secondarysynchronization channel when two short sequences are mapped to afrequency domain in a localization form, and FIG. 3 is a diagramillustrating a configuration of a secondary synchronization channel whentwo short sequences are mapped to a frequency domain in a distributionform.

Referring to FIG. 2 to FIG. 3, a secondary synchronization signal, whichis inserted into a secondary synchronization channel, according to anexemplary embodiment of the present invention is formed by combining twoshort sequences. Cell group information and frame synchronizationinformation are mapped to the two short sequences.

As shown in FIG. 2, a first short sequence may be locally allocated tosub-carriers, and then the second short sequence may be locallyallocated to remaining sub-carriers. In addition, as shown in FIG. 3,the first short sequence may be allocated to every even-numberedsub-carriers (n=0, 2, 4, . . . , 60), and the second short sequence maybe allocated to every odd-numbered sub-carrier (n=1, 3, 5, . . . , 61).

The short sequence length corresponds to half of the number ofsub-carriers allocated to the secondary synchronization channel. Thatis, the number of short sequence elements that can be generated is up tohalf of the number of sub-carriers allocated to the secondarysynchronization channel. For instance, when the number of sub-carriersallocated to the secondary synchronization channel is 62, the shortsequence length corresponds to 31 and the number of short sequenceelements that can be generated is up to 31.

Since two short sequences are allocated to each secondarysynchronization channel, the number of secondary synchronizationsequences generated by a combination of two short sequences is 961(=31×31) at maximum. However, since the information that should beincluded in the secondary synchronization channel is cell groupinformation and frame boundary information, 170 or 340 (=170×2)secondary synchronization sequences are required. Accordingly, thenumber 961 is a sufficiently large value in comparison with the number170 or 340.

Next, an apparatus for generating a downlink frame according to anexemplary embodiment of the present invention will be described withreference to FIG. 4. FIG. 4 is a block diagram of the apparatus forgenerating the downlink frame according to the exemplary embodiment ofthe present invention.

As shown in FIG. 4, the apparatus for generating the downlink frameaccording to the exemplary embodiment of the present invention includesa sequence generating unit 410, a synchronization signal generating unit420, a frequency mapping unit 430, and an OFDM transmitting unit 440.

The sequence generating unit 410 generates a sequence for acquiring timeand frequency synchronization, a cell identification sequence, aplurality of short sequences, and a scrambling sequence for reducingadjacent cell interference, respectively, and transmits them to thesynchronization signal generating unit 420.

The synchronization signal generating unit 420 generates a primarysynchronization signal, a secondary synchronization signal, and a pilotpattern by using sequences received from the sequence generating unit410.

The synchronization signal generating unit 420 generates the primarysynchronization signal by using the sequence for acquiring time andfrequency synchronization and the cell identification sequence. Inaddition, the synchronization signal generating unit 420 generates thesecondary synchronization signal by using the plurality of shortsequences and the scrambling sequences for reducing adjacent cellinterference.

The synchronization signal generating unit 420 generates the pilotpattern of downlink signals by allocating a unique scrambling sequenceallocated to each cell for encoding a common pilot symbol and datasymbol of a cellular system to the pilot channel.

The frequency mapping unit 430 generates the downlink frame by mappingthe primary synchronization signal, the secondary synchronizationsignal, and the pilot pattern that are generated from thesynchronization signal generating unit 420 and frame control informationand transmission traffic data that are transmitted from external sourcesto the time and frequency domains.

The OFDM transmitting unit 440 receives the downlink frame from thefrequency mapping unit 430 and transmits the downlink frame throughgiven transmission antenna.

Referring to FIG. 5 to FIG. 8, a method of generating a downlink frameaccording to an exemplary embodiment of the present invention will bedescribed. FIG. 5 is a flowchart illustrating the method of generatingthe downlink frame according to the exemplary embodiment of the presentinvention.

As shown in FIG. 5, the sequence generating unit 410 generates aplurality of short sequences and a plurality of scrambling sequences forreducing interference of a plurality of adjacent cells and transmitsthem to the synchronization signal generating unit 420 (S510).

The synchronization signal generating unit 420 generates a secondarysynchronization signal by using the short sequences and the scramblingsequences for reducing interference of the plurality of adjacent cellsreceived from the sequence generating unit 410 (S520). In the exemplaryembodiment of the present invention, it is described that one frameincludes two secondary synchronization channels. However, it is notlimited thereto.

Referring to FIG. 6 to FIG. 8, three different methods of generating asecondary synchronization signal according to an exemplary embodiment ofthe present invention will be described. FIG. 6 is a diagramillustrating the first method of generating a secondary synchronizationsignal according to the exemplary embodiment of the present invention,FIG. 7 is a diagram illustrating the second method of generating asecondary synchronization signal according to the exemplary embodimentof the present invention, and FIG. 8 is a diagram illustrating the thirdmethod of generating a secondary synchronization signal according to theexemplary embodiment of the present invention.

A short sequence (wn) is a binary sequence (or binary code) representingcell group information. That is, the short sequence (wn) is the binarysequence allocated to a cell group number and frame synchronization.Moreover, the length of the short sequence corresponds to half of thenumber of sub-carriers allocated to the secondary synchronizationchannel. In the exemplary embodiment of the present invention, it isdescribed that the number of sub-carriers allocated to the secondarysynchronization channel is 62. However, it is not limited thereto.Accordingly, the short sequence length according to the exemplaryembodiment of the present invention is 31.

The first short sequence w0 is allocated to even-numbered sub-carriersof the first secondary synchronization channel and is defined as givenin Equation 1.

w0=[w0(0), w0(1), . . . , w0(k), . . . , w0(30)]  (Equation 1)

Here, k denotes an index of the even-numbered sub-carriers used for asecondary synchronization channel.

The second short sequence w1 is allocated to odd-numbered sub-carriersof the first secondary synchronization channel and is defined as givenin Equation 2.

w1=[w1(0), w1(1), . . . , w1(m), . . . , w1(30)]  (Equation 2)

Here, m denotes an index of the odd-numbered sub-carriers used for thesecondary synchronization channel.

The third short sequence w2 is allocated to even-numbered sub-carriersof the second secondary synchronization channel and is defined as givenin Equation 3.

w2=[w2(0), w2(1), . . . , w2(k), . . . , w2(30)]  (Equation 3)

The fourth short sequence w3 is allocated to odd-numbered sub-carriersof the second secondary synchronization channel and is defined as givenin Equation 4.

w3=[w3(0), w3(1), . . . , w3(m), . . . , w3(30)]  (Equation 4)

Here, the short sequences w0, w1, w2, and w3 may be different sequences.In addition, the relationship between the short sequences w0, w1, w2,and w3 may be represented as w0=w3 and w1=w2 (or w0=w2 and w1=w3). Giventhat w0=w3 and w1=w2, then the pattern of short sequences allocated tothe second secondary synchronization channel can be determined onlythrough the pattern of short sequences allocated to the first secondarysynchronization channel. Accordingly, by storing only 170 secondarysynchronization sequences generated by a combination of two shortsequences allocated to the first secondary synchronization channel, amobile station can reduce the complexity needed to obtaining the cellgroup information and frame boundary information.

According to the first method of generating a secondary synchronizationsignal as shown in FIG. 6, the first short sequence is allocated toevery even-numbered sub-carrier of the first secondary synchronizationchannel and the second short sequence is allocated to every odd-numberedsub-carrier of the first secondary synchronization channel. In addition,the third short sequence is allocated to every even-numbered sub-carrierof the second secondary synchronization channel and the fourth shortsequence is allocated to every odd-numbered sub-carrier of the secondsecondary synchronization channel.

According to the first method of generating the secondarysynchronization signal, the secondary synchronization signal is formedby a combination of two short sequences having the length of 31.Accordingly, the number of secondary synchronization signals is 961which is a sufficiently large value in comparison with the number 170 or340.

According to the second method of generating the secondarysynchronization signal shown in FIG. 7, a first sequence determined byEquation 5 is allocated to every even-numbered sub-carrier of the firstsecondary synchronization channel(slot 0), and a second sequencedetermined by Equation 6 is allocated to every odd-numbered sub-carrierof the first secondary synchronization channel(slot 0). In addition, athird sequence determined by Equation 7 is allocated to everyeven-numbered sub-carrier of the second secondary synchronizationchannel(slot 10), and a fourth sequence determined by Equation 8 isallocated to every odd-numbered sub-carrier of the second secondarysynchronization channel(slot 10).

A scrambling sequence P_(j,0,1) scrambling the first short sequence w0is defined by P_(j,0,1)=[P_(j,0,1)(0), P_(j,0,1)(1), . . . ,P_(j,0,1)(k), . . . , P_(j,0,1)(30)], where j (j=0, 1, 2) is the numberof the cell identification sequence allocated to the primarysynchronization channel. Accordingly, the scrambling sequence P_(j,0,1)is determined by the primary synchronization signal. The scramblingsequence P_(j,0,1) is a known value when a sequence is demapped to finda cell ID group and a frame boundary in the mobile station.

As indicated in Equation 5, each element of a first sequence c₀according to the second method of generating the secondarysynchronization signal is a product of each element of the first shortsequence w0 and each element of the scrambling sequence P_(j,0,1)corresponding thereto.

c₀=[w0(0)P_(j,0,1)(0), w0(1)P_(j,0,1)(1), . . . , w0(k)P_(j,0,1)(k), . .. , w0(30)P_(j,0,1)(30)]  (Equation 5)

Here, k denotes an index of the even-numbered sub-carriers used for thesecondary synchronization channel.

The scrambling sequence scrambling the second short sequence w1 isP_(j,1,1) and S_(w0).

The scrambling sequence P_(j,1,1) is P_(j,1,1)=[P_(j,1,1)(0),P_(j,1,1)(1), . . . , P_(j,1,1)(m), . . . , P_(j,1,1)(30)], where j(j=0, 1, 2) is the number of the cell identification sequence allocatedto the primary synchronization channel. Accordingly, the scramblingsequence P_(j,1,1) is determined by the primary synchronization signal.In addition, the scrambling sequence P_(j,1,1) may be the same as thescrambling sequence P_(j,0,1) or may be different from the scramblingsequence P_(j,0,1). When the scrambling sequence P_(j,1,1) is differentfrom the scrambling sequence P_(j,0,1), it can be possible to reduceinterference.

The scrambling sequence P_(j,1,1) is a previously known value when asequence is demapped to find a cell ID group and a frame boundary in themobile station.

In addition, the scrambling sequence S_(w0) is S_(w0)=[S_(w0)(0),S_(w0)(1), . . . , S_(w0)(m), . . . , S_(w0)(30)], and the scramblingsequence S_(w0) is determined by the first short sequence w0.

At this time, a plurality of short sequences are grouped into aplurality of short sequence group and the S_(w0) may be determined by ashort sequence group to which the first short sequence is assigned.

For example, according to the exemplary embodiment of the presentinvention, since the length of the first short sequence is 31, there are31 short sequences. Accordingly, by assigning the short sequences Nos.0-7 to the group 0, the short sequences Nos. 8-15 to the group 1, theshort sequences Nos. 16-23 to the group 2, and the short sequences Nos.24-30 to the group 3. Accordingly S_(w0) is determined by mapping alength-31 scrambling code to the group to which the first short sequencenumber is assigned.

Furthermore, 31 short sequences may be classified into eight groups bygrouping the numbers of the first short sequences having the identicalremainder when we divide each number of short sequences by 8. That is,by assigning the short sequence number having the remainder of 0 whendividing the short sequence numbers by 8 to the group 0, the shortsequence having the remainder of 1 when dividing the short sequencenumbers by 8 to the group 1, the short sequence having the remainder of2 when dividing the short sequence numbers by 8 to the group 2, theshort sequence having the remainder of 3 when dividing the shortsequence numbers by 8 to the group 3, the short sequence having theremainder of 4 when dividing the short sequence numbers by 8 to thegroup 4, the short sequence having the remainder of 5 when dividing theshort sequence numbers by 8 to the group 5, the short sequence havingthe remainder of 6 when dividing the short sequence numbers by 8 to thegroup 6, and the short sequence having the remainder of 7 when dividingthe short sequence numbers by 8 to the group 7. Accordingly S_(w0) isdetermined by mapping a length-31 scrambling code to the group to whichthe first short sequence number is assigned.

As indicated in Equation 6, each element of a second sequence c₁according to the second method of generating the secondarysynchronization signal is a product of each element of the second shortsequence w1 and each element of the scrambling sequences P_(j,1,1) andS_(w0) corresponding thereto.

c₁=[w1(0)S_(w0)(0)P_(j,1,1)(0), w1(1)S_(w0)(1)P_(j,1,1)(1), . . . ,w1(m)S_(w0)(m)P_(j,1,1)(m), . . . ,w1(30)S_(w0)(30)P_(j,1,1)(30)]  (Equation 6)

Herein, m denotes the index of odd-numbered sub-carriers used for thesecondary synchronization channel.

A scrambling sequence P_(j,0,2) for scrambling a third short sequence w2is P_(j,0,2)=[P_(j,0,2)(0), P_(j,0,2)(1), . . . , P_(j,0,2)(k) . . . ,P_(j,0,2)(30)], where j (j=0, 1, 2) is the number of the cellidentification sequence allocated to the primary synchronizationchannel. Accordingly, the scrambling sequence P_(j,0,2) is determined bythe primary synchronization signal. In addition, the scrambling sequenceP_(j,0,2) is a previously known value when the sequence is demapped tofind the cell ID group and frame boundary in the mobile station.

As indicated in Equation 7, each element of a third sequence c₂according to the second method of generating the secondarysynchronization signal is a product of each element of the third shortsequence w2 and each element of the scrambling sequence P_(j,0,2)corresponding thereto.

c₂=[w2(0)P_(j,0,2)(0), w2(1)P_(j,0,2)(1), . . . , w2(k)P_(j,0,2)(k), . .. , w2(30)P_(j,0,2)(30)]  (Equation 7)

Herein, k denotes the index of even-numbered sub-carriers used for thesecondary synchronization channel.

Scrambling sequences for scrambling a fourth short sequence areP_(j,1,2) and S_(w2).

The scrambling sequence P_(j,1,2) is P_(j,1,2)=[P_(j,1,2)(0),P_(j,1,2)(1), . . . , P_(j,1,2)(m), . . . , P_(j,1,2)(30)], and j (j=0,1, 2) is the number of the cell identification sequence allocated to theprimary synchronization channel. Accordingly, the scrambling sequenceP_(j,1,2) is determined by the primary synchronization signal. Thescrambling sequence P_(j,1,2) is a previously known value when asequence is demapped to find the cell ID group and frame boundary in themobile station.

Furthermore, the scrambling sequence S_(w2) is S_(w2)=[S_(w2)(0),S_(w2)(1), S_(w2)(m), . . . , S_(w2)(30)], and the scrambling sequenceS_(w2) is determined by the third short sequence w2.

At this time, the S_(w2) may be determined by a short sequence group towhich the third short sequence is assigned by grouping short sequences.

For example, according to the exemplary embodiment of the presentinvention, since the length of the third short sequence is 31 as well,there are 31 short sequences. Accordingly, by assigning the shortsequences Nos. 0-7 to the group 0, the short sequences Nos. 8-15 to thegroup 1, the short sequences Nos. 16-23 to the group 2, and the shortsequences Nos. 24-30 to the group 3. Accordingly S_(w2) is determined bymapping a length-31 scrambling code to the group to which the thirdshort sequence number is assigned.

Furthermore, 31 short sequences may be classified into eight groups bygrouping the numbers of the third short sequences having the identicalremainder when we divide each number of short sequences by 8. That is,by assigning the short sequence number having the remainder of 0 whendividing the short sequence numbers by 8 to the group 0, the shortsequence having the remainder of 1 when dividing the short sequencenumbers by 8 to the group 1, the short sequence having the remainder of2 when dividing the short sequence numbers by 8 to the group 2, theshort sequence having the remainder of 3 when dividing the shortsequence numbers by 8 to the group 3, the short sequence having theremainder of 4 when dividing the short sequence numbers by 8 to thegroup 4, the short sequence having the remainder of 5 when dividing theshort sequence numbers by 8 to the group 5, the short sequence havingthe remainder of 6 when dividing the short sequence numbers by 8 to thegroup 6, and the short sequence having the remainder of 7 when dividingthe short sequence numbers by 8 to the group 7. Accordingly S_(w2) isdetermined by mapping a length-31 scrambling code to the group to whichthe third short sequence number is assigned.

As indicated in Equation 8, each element of a fourth sequence c₃according to the second method of generating the secondarysynchronization signal is a product of each element of the fourth shortsequence w3 and each element of the scrambling sequences P_(j,1,2) andS_(w2) corresponding thereto.

c₃=[w3(0)S_(w2)(0)P_(j,1,2)(0), w3(1)S_(w2)(1)P_(j,1,2)(1), . . . ,w3(m)S_(w2)(m)P_(j,1,2)(m), . . . ,w3(30)S_(w2)(30)P_(j,1,2)(30)]  (Equation 8)

Herein, m denotes the index of odd-numbered sub-carriers used for thesecondary synchronization channel.

Here, the relationship between the scrambling sequences and the shortsequences may be set as P_(j,0,1)=P_(j,0,2), P_(j,1,1)=P_(j,1,2),P_(j,0,1)≠P_(j,1,1), P_(j,0,2)≠P_(j,1,2), and w0≠w1≠w2≠w3 (or w0=w3 andw1=w2). In this case, the cell group and frame identify information aremapped to the combination of the first to fourth short sequences, andthe number of descrambling hypotheses in the mobile station with respectto the scrambling of secondary synchronization channel determined by thecell identification sequence number of the primary synchronizationchannel is reduced to 3.

Furthermore, the relationship between the scrambling sequences and theshort sequences may be set as P_(j,0,1)≠P_(j,0,2), P_(j,1,1)≠P_(j,1,2),P_(j,0,1)≠P_(j,1,1), P_(j,0,2)≠P_(j,1,2), w0=w2, and w1=w3. In thiscase, the cell group information is mapped to the combination of thefirst short sequence and the second short sequence, and the framesynchronization information is mapped to the scrambling sequences(P_(j,0,1), P_(j,0,2), P_(j,1,1), P_(j,1,2)) of the secondarysynchronization channel determined by the cell identification sequencenumber of the primary synchronization channel. Then, the number ofdescrambling hypotheses of the mobile station with respect to thescrambling of the secondary synchronization channel determined by thecell identification sequence number of the primary synchronizationchannel is increased to 6. However, the combination number of the cellgroup identification sequences is reduced to half, and the number ofdescrambling hypotheses of the mobile station with respect to thescrambling determined by the first and third short sequences is alsoreduced to half.

As shown in FIG. 8, in the third method of generating a secondarysynchronization signal, a first sequence determined by Equation 9 isallocated to every even-numbered sub-carrier of a first secondarysynchronization channel, and a second sequence determined by Equation 10is allocated to every odd-numbered sub-carrier of the first secondarysynchronization channel. Moreover, a third sequence determined byEquation 11 is allocated to every even-numbered sub-carrier of a secondsecondary synchronization channel, and a fourth sequence determined byEquation 12 is allocated to every odd-numbered sub-carrier of the secondsecondary synchronization channel.

That is, according to the second method of generating the secondarysynchronization signal, the first short sequence is scrambled with afirst scrambling sequence having the length of 31, which is determinedby the cell identification sequence allocated to the primarysynchronization channel, and the second short sequence is scrambled witha second scrambling sequence having the length of 31, which isdetermined by the cell identification sequence allocated to the primarysynchronization channel. However, according to the third method ofgenerating the secondary synchronization signal, the first shortsequence and the second short sequence are scrambled with a scramblingsequence having the length of 62, which is determined by the cellidentification sequence allocated to the primary synchronizationchannel.

P_(j,1) is the scrambling sequence that scrambles the first shortsequence and the second short sequence, and P_(j,2) is the scramblingsequence that scrambles the third short sequence and the fourth shortsequence. The scrambling sequences P_(j,1) and P_(j,2) are representedas P_(j,1)=[P_(j,1)(0), P_(j,1)(1), . . . , P_(j,1)(k), . . . ,P_(j,1)(61)], and P_(j,2)=[P_(j,2)(0), P_(j,2)(1), . . . , P_(j,2)(k), .. . , P_(j,2)(61)].

Here, j (j=0, 1, 2) is the number of the cell identification sequenceallocated to the primary synchronization channel. Accordingly, thescrambling sequences P_(j,1) and P_(j,2) are determined by the number ofthe cell identification sequence allocated to the primarysynchronization channel.

According to the third method of generating the secondarysynchronization signal, the first sequence c₀ is as indicated inEquation 9, the second sequence c₁ is as indicated in Equation 10, thethird sequence c₂ is as indicated in Equation 11, and the fourthsequence c₃ is as indicated in Equation 12.

c₀=[w0(0)P_(j,1)(0), w0(1)P_(j,1)(1), . . . , w0(k)P_(j,1)(k), . . . ,w0(30)P_(j,1)(30)]  (Equation 9)

c₁=[w1(0)S_(w0)(0)P_(j,1)(31), . . . , w1(1)S_(w0)(1)P_(j,1)(32),w1(m)S_(w0)(m)P_(j,1)(31+m), . . . ,w1(30)S_(w0)(30)P_(j,1)(61)]  (Equation 10)

c₂=[w2(0)P_(j,2)(0), w2(1)P_(j,2)(1), . . . , w2(k)P_(j,2)(k), . . . ,w2(30)P_(j,2)(30)]  (Equation 11)

C₃=[w3(0)S_(w2)(0)P_(j,2)(31), w3(1)S_(w2)(1)P_(j,2)(32), . . . ,w3(m)S_(w2)(m)P_(j,2)(31+m), . . . ,w3(30)S_(w2)(30)P_(j,2)(61)]  (Equation 12)

In Equation 9 to Equation 12, k denotes the index of the even-numberedsub-carriers to be used for the secondary synchronization channel, and mdenotes the index of the odd-numbered sub-carriers to be used for thesecondary synchronization channel.

The frequency mapping unit 430 generates the downlink frame by mappingthe secondary synchronization signal that are generated from thesynchronization signal generating unit 420, and transmission trafficdata to the time and frequency domains S530.

The OFDM transmitting unit 440 receives the downlink frame from thefrequency mapping unit 430 and transmits the downlink frame throughgiven transmission antenna S540.

A method of searching cells by the mobile station by using the downlinkframe generated by the exemplary embodiment of the present inventionwill now be described with reference to FIG. 9 and FIG. 11.

FIG. 9 is a block diagram of an apparatus for searching cells accordingto the exemplary embodiment of the present invention, FIG. 10 is aflowchart illustrating a cell searching method according to a firstexemplary embodiment of the present invention, and FIG. 11 is aflowchart illustrating a cell searching method according to a secondexemplary embodiment of the present invention.

As shown in FIG. 9, the apparatus for searching the cells according tothe exemplary embodiment of the present invention includes a receivingunit 710, a symbol synchronization estimating and frequency offsetcompensating unit 720, a Fourier transforming unit 730, and a cell IDestimating unit 740.

A cell searching method according to the first exemplary embodiment ofthe present invention will now be described with reference to FIG. 10.

As shown in FIG. 10, the receiving unit 710 receives the framestransmitted from the base station, and the symbol synchronizationestimating and frequency offset compensating unit 720 filters thereceived signal by as much as a bandwidth allocated to thesynchronization channel and acquires the symbol synchronization byrespectively correlating the filtered received signal and a plurality ofknown primary synchronization signals, and compensates the frequencyoffset by estimating frequency synchronization (S810). The symbolsynchronization estimating and frequency offset compensating unit 720respectively correlates the filtered received signal and the pluralityof known primary synchronization signals and estimates a time of thelargest correlation value as the symbol synchronization, and transmits anumber of a primary synchronization signal having the largestcorrelation value to the cell ID estimating unit 740. At this time, thefrequency offset may be compensated in the frequency domain afterperforming the Fourier transform.

The Fourier transforming unit 730 performs Fourier transform of thereceived signals on the basis of the symbol synchronization estimated bythe symbol synchronization estimating and frequency offset compensatingunit 720 (S820).

The cell ID estimating unit 740 estimates a cell ID group and framesynchronization by respectively correlating the Fourier transformedreceived signal with a plurality of known secondary synchronizationsignals S830. The cell ID estimating unit 740 respectively correlates aplurality of secondary synchronization signals with the Fouriertransformed received signal, and estimates the frame synchronization andthe cell ID group by using a secondary synchronization signal that hasthe largest correlation value. Herein, the plurality of secondarysynchronization signals are given by applying P_(j,0,1), P_(j,0,2),P_(j,1,1) and P_(j,1,2) that are determined in accordance with a primarysynchronization signal that corresponds to the number of a primarysynchronization signal transmitted from the symbol synchronizationestimating and frequency offset compensating unit 720 to Equation 5 toEquation 8, At this time, in the case that a synchronization channelsymbol exists in one slot or one OFDM symbol within one frame, thesymbol synchronization becomes frame synchronization, and therefore, itis not necessary to additionally acquire frame synchronization.

In addition, the cell ID estimating unit 740 estimates cell IDs by usingthe number of a primary synchronization signal transmitted from thesymbol synchronization estimating and frequency offset compensating unit720 and the estimated cell ID group S840. At this time, the cell IDestimating unit 740 estimates the cell ID with reference to a knownmapping relationship between cell ID, the cell ID group, and a number ofprimary synchronization signal.

The estimated cell ID information may be verified by using scramblingsequence information included in the pilot symbol duration.

A cell searching method according to the second exemplary embodiment ofthe present invention will now be described with reference to FIG. 11.

As shown in FIG. 11, the receiving unit 710 receives a frame transmittedfrom the base station, and the symbol synchronization estimating andfrequency offset compensating unit 720 filters the received signal by asmuch as a bandwidth allocated to the synchronization channel andacquires the symbol synchronization by respectively correlating thefiltered received signal and a plurality of known primarysynchronization signals, and compensates the frequency offset byestimating frequency synchronization S910. The symbol synchronizationestimating and frequency offset compensating unit 720 respectivelycorrelates the filtered received signal and the plurality of knownprimary synchronization signals and estimates a time of the largestcorrelation value as the symbol synchronization, and transmits aplurality of correlation values of the plurality of known primarysynchronization signals and filtered received signal to the cell IDestimating unit 740. At this time, the frequency offset compensation maybe performed in the frequency domain after Fourier-transformed.

The Fourier transforming unit 730 Fourier-transforms the received signalwith reference to the symbol synchronization that is estimated by thesymbol synchronization estimating and frequency offset compensating unit720 S920.

The cell ID estimating unit 740 estimates cell IDs by using theplurality of correlation values transmitted from the symbolsynchronization estimating and frequency offset compensating unit 720,and correlation values of the Fourier-transformed received signal and aplurality of known secondary synchronization signals S930. The cell IDestimating unit searches a secondary synchronization signal having thelargest correlation value by correlating each of the plurality of knownsecondary synchronization signals with the Fourier-transformed receivedsignal for each of the plurality of known primary synchronizationsignals. Here, the plurality of secondary synchronization signals aregiven by applying P_(j,0,1), P_(j,0,2), P_(j,1,1) and P_(j,1,2) that aredetermined in accordance with the corresponding primary synchronizationsignal to Equation 5 to Equation 8.

In addition, the cell ID estimating unit 740 combines the correlationvalue of each known primary synchronization signal transmitted from thesymbol synchronization estimating and frequency offset compensating unit720 and the correlation value of the secondary synchronization signalhaving the largest correlation value for each of the plurality of knownprimary synchronization signals.

The cell ID estimating unit 740 estimates frame synchronization and acell ID group by using a secondary synchronization signal having thelargest combined value among the combined values of the correlationvalues of a primary synchronization signal and a secondarysynchronization signal. In addition, the cell ID estimating unit 740estimates a cell ID by using the primary synchronization signal havingthe largest combined value and the estimated cell ID group. At thistime, the cell ID estimating unit 740 estimates the cell ID withreference to a known mapping relationship between the cell ID group,cell ID and the primary synchronization signal number.

The exemplary embodiment of the present invention can be not onlyimplemented by the above-described apparatus and/or method, but can beimplemented by, for example, a program that achieves the functioncorresponding to the configuration of the exemplary embodiment of thepresent invention and a recording medium in which the program isrecorded. This will be easily implemented from the above-describedexemplary embodiment of the present invention by those skilled in therelated art.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of generating a downlink frame including a primarysynchronization signal and a secondary synchronization signal,comprising: generating a first short sequence and a second shortsequence indicating cell group information; generating a firstscrambling sequence and a second scrambling sequence determined by theprimary synchronization signal; generating a third scrambling sequencedetermined by the first short sequence; scrambling the first shortsequence with the first scrambling sequence and scrambling the secondshort sequence with the second scrambling sequence and the thirdscrambling sequence; and mapping the secondary synchronization signalthat includes the scrambled first short sequence and the scrambledsecond short sequence to a frequency domain.
 2. The method of claim 1,wherein the mapping of the secondary synchronization signal includesalternately disposing the scrambled first short sequence and thescrambled second short sequence on a plurality of sub-carriers.
 3. Themethod of claim 2, wherein the first scrambling sequence and the secondscrambling sequence are different from each other.
 4. The method ofclaim 2, wherein: a plurality of cells are grouped into a plurality ofcell groups, each of the cell groups including at least two cells; theplurality of cell groups are distinguished by cell group numbers; atleast two cells in each of the cell groups are distinguished by thenumbers of cell identification sequences allocated to the primarysynchronization signal; and the first short sequence and the secondshort sequence correspond to the cell group numbers.
 5. The method ofclaim 2, wherein: the downlink frame includes a plurality of slots, eachslot having a plurality of symbols; and the primary synchronizationsignal is located on a last symbol of a slot, and the secondarysynchronization signal is located on a symbol right ahead of the lastsymbol of the slot.
 6. An apparatus for generating a downlink frameincluding a primary synchronization signal and a secondarysynchronization signal, comprising: a sequence generating unit thatgenerates a first short sequence and a second short sequence indicatingcell group information, the first scrambling sequence and the secondscrambling sequence determined by the primary synchronization signal,and a third scrambling sequence determined by the first short sequence;and a synchronization signal generating unit that scrambles the firstshort sequence with the first scrambling sequence and scrambles thesecond short sequence with the second scrambling sequence and the thirdscrambling sequence, and then generates a secondary synchronizationsignal including the scrambled first short sequence and the scrambledsecond short sequence, respectively.
 7. The apparatus of claim 6,further comprising a frequency mapping unit that alternately disposesthe scrambled first short sequence and the scrambled second shortsequence on a plurality of sub-carriers.
 8. The apparatus of claim 7,wherein the first scrambling sequence and the second scrambling sequenceare different from each other.
 9. The apparatus of claim 7, wherein: aplurality of cells are grouped into a plurality of cell groups, each ofthe cell groups including at least two cells; the plurality of cellgroups are distinguished by cell group numbers; at least two cells ineach of the cell groups are distinguished by the numbers of cellidentification sequences allocated to the primary synchronizationsignal; and the first short sequence and the second short sequencecorrespond to the cell group numbers.
 10. The apparatus of claim 7,wherein: the downlink frame includes a plurality of slots, each slothaving a plurality of symbols; and the primary synchronization signal islocated on a last symbol of a slot, and the secondary synchronizationsignal is located on a symbol right ahead of the last symbol of theslot.
 11. A method of searching a cell by a mobile station, comprising:receiving a downlink frame including a primary synchronization signaland a secondary synchronization signal; and identifying a cell by usingthe primary synchronization signal and the secondary synchronizationsignal, wherein, in the downlink frame, a first short sequence scrambledwith a first scrambling sequence and a second short sequence scrambledwith a second scrambling sequence and a third scrambling sequence arealternately disposed on a plurality of sub-carriers, and the first shortsequence and the second short sequence indicate cell group information,the first scrambling sequence and the second scrambling sequence aredetermined by the primary synchronization signal, and the thirdscrambling sequence is determined by the first short sequence.
 12. Themethod of claim 11, wherein the first scrambling sequence and the secondscrambling sequence are different from each other.
 13. The method ofclaim 11, wherein the identifying of the cell comprises: identifying acell group by using the secondary synchronization signal; andidentifying the cell in the cell group by using the primarysynchronization signal.
 14. The method of claim 11, wherein; thedownlink frame includes a plurality of slots, each slot having aplurality of symbols; and the primary synchronization signal is locatedon a last symbol of a slot, and the secondary synchronization signal islocated on a symbol right ahead of the last symbol of the slot.
 15. Anapparatus for searching a cell by a mobile station, comprising: areceiving unit that receives a downlink frame including a primarysynchronization signal and a secondary synchronization signal; and acell ID estimating unit that identifies a cell by using the primarysynchronization signal and the secondary synchronization signal,wherein, in the downlink frame, a first short sequence scrambled with afirst scrambling sequence and a second short sequence scrambled with asecond scrambling sequence and a third scrambling sequence arealternately disposed on a plurality of sub-carriers, and the first shortsequence and the second short sequence indicate cell group information,the first scrambling sequence and the second scrambling sequence aredetermined by the primary synchronization signal, and the thirdscrambling sequence is determined by the first short sequence.
 16. Theapparatus of claim 15, wherein the first scrambling sequence and thesecond scrambling sequence are different from each other.
 17. Theapparatus of claim 15, wherein: the downlink frame includes a pluralityof slots, each slot having a plurality of symbols; and the primarysynchronization signal is located on a last symbol of a slot, and thesecondary synchronization signal is located on a symbol right ahead ofthe last symbol of the slot.